Strip line wave guide coupler



United States Patent "ice 3,094,677 STRIP LINE WAVE GUIDE COUPLER EugeneJ. Theriot, Greensboro, N.C., assignor to Bell Telephone Laboratories,Incorporated, New York, N.Y., a corporation of New York Filed Dec. 19,1960, Ser. No. 76,643 5 Claims. (Cl. 333-) This invention relates toelectromagnetic wave transmission and more particularly to directionalcouplers of the planar or strip line transmission line type.

There has developed in the art, as exemplified by printed circuitnetworks, a wide area in which thin flat conductive members separated bydielectric media have taken the place of the relatively bulky andcumbersome hollow pipe and circularly coaxial wave transmission lines.Broadly speaking, these planar trans-mission lines fall into twoclasses. The first class is one in which waves propagate along a narrowflat strip conductor dielectrically separated from a single widerconductive ground plane. These lines are commonly known as Microstrip,and are characterized by the transmission properties set out in anarticle by Maurice Arditi entitled Characteristics and Applications ofMicrostrip for Microwave Wiring, beginning at page 31 of volume MTT-3 ofIRE Transactions on Microwave Theory and Techniques (March 1955).Certain properties of Microstrip directional couplers are disclosed andclaimed in United States Patent 2,951,218, issued August 30, 1960, toMr. Arditi.

The second class of planar transmission lines comprises at least atriple conductor structure having one or more flat strip conductorscentrally positioned between and dielectrically spaced away from a pairof outer conductive ground planes. These strip lines are electricallysimilar to the well-known coaxial lines. It is toward this second classof strip transmission lines that the present invention is directed, andto which the use herein-after of the terms strip transmission line andstrip line is intended to refer.

It is now well known that energy propagating along a first, or main,strip line may be directionally coupled onto a second, or auxiliary,strip line by bringing the center strip conductors of the auxiliary andthe main lines into proximity. The amount of the coupling as well as itsdirectivity will be determined by the physical parameters associatedwith the coupled lines.

It is oftentimes desirable that small adjustments in the amount ofcoupling between adjacent strip lines he made. The process foraccomplishing this has been, however, crude and inexact. When, forexample, the strip line comprised a pair of mirror image circuitsprinted on dielectric board the procedure involved assembling the unit,measuring the coupling, disassembling the unit, removing, or adding, asmall segment from, or to, the center conductor, and reassembling andretesting the unit. Oftentimes a single unit required a plurality ofadjusting operations since it was diflicult to vary the width of thecenter strip the precise amount. Furthermore, the reassembled unit oftenhad an alignment of parts slightly different from the originalalignment, thus necessitating further adjustment.

-It is, therefore, an object of the present invention to adjust thecoupling of strip line directional couplers in a simple and precisemanner.

It is a further object of the invention to adjust the coupling of stripline directional couplers by means externally accessible and readilyavailable.

In accordance with the present invention it has been discovered that theamount of coupling between parallel path strip lines may be increased byremoving a portion 3,094,677 Patented June 18, 1963 of one or bothground planes in the vicinity of the adjacent strips. Reduced couplingmay then be afforded by conductively covering an area of the removedground plane. In this manner the amount of coupling in a strip linedirectional coupler may be easily controlled and trimmed to any desiredvalue. The invention is particularly applicable in plural circuitcouplers in which it is desired that a plurality of auxiliary couplingcircuits couple identical amounts of energy from a main transmissioncircuit.

Thus, a further object of the invention is to adjust a plurality ofstrip line coupling circuits to couple equal amounts of wave energy froma main strip transmission line.

In accordance with a principal embodiment of the invention, first andsecond flat conductive strips are positioned in coupling proximitybet-ween a pair of conductive ground planes with a nonconductiveaperture located in one of the ground planes in the area between theadjacent strips. A conductive member is mounted in superposedrelationship to the aperture and is variably positionable over theaperture to afiord control over the coupling strength between the lines.

The above and other objects of the invention, together with its variousfeatures, will be more readily understandable by reference to theaccompanying drawing in which:

FIG. 1 is a partially beoken away perspective view of a strip linedirectional coupler in accordance with the invention;

FIGS. 2 and 3 are transverse cross sectional views of strip linecouplers given tor purposes of explanation; and

FIG. 4 is a top view of a multici-rcuit strip line coupler incorporatingthe principles of the present invention.

Referring more particularly to FIG. 1, there is shown a strip linedirectional coupler 10 in accordance with the present invention. Theenergy transmission paths comprise two parallel conductive strips 11, 12spaced apart in the same plane and two conductive planes 13, 14 lying onopposite sides of said strips and in planes parallel to the common planeof said strips. The volume between strips 11, 12 and planes 13, 14 isoccupied by slabs 15, 16 of low loss dielectric material such asTeflon-fiberglass, polystyrene or polyethylene, in accordance withwell-known strip line techniques. The centrally positioned stripfunctions in a manner analogous to the center conductor of a circularlycoaxial line. The two ground planes 13, 14 are analogous to the outerconductor of a coaxial line. In order that the coupler 10 be dominantmode and thus support only the lowest order strip line transmission mode(TEM), certain limitations must be met. First, the spacing betweenground planes 13, 14 as well as the Wider electrical dimension of strips11, 12 must be less than one-half wavelength at the operating frequencyand second, the central strips 11, 12 must be substantially centeredbetween and substantially parallel to outer planes 13, 14. If either theground plane spacing or the electrical width of the center stripsexceeds one-half wavelength, conversion of power in the dominant stripline mode to higher order strip line modes may occur. A typicalfabrication method for a strip transmission lines involves conductivelyprint ing center strips corresponding to strips 11, 12 on one side of afirst dielectric board, fabricating a second dielectric board which isthe mirror image of said first board, positioning these boards face toface and securing them in such position. Theouter surfaces of the twoboards are conductively coated and thus form ground planes correspondingto planes 13, 14.

Positioned within plane 13 in FIG. 1 is elongated aperture or slot 17,which extends completely through plane 13 and thus forms a nonconductivearea in that ground plane.

Slot 17 is located in that portion of plane 13 which overlays the areabetween strips 11, 12. As illustrated in FIG. 1, the slot is positionedcloser to strip 12 than to strip 11, and, with respect to an imaginarycenterline between the strips, is completely on that side of thecenterline toward strip 12. In general, the slot 17 should have a widthequal to approximately one-half the spacing between the parallel stripsto be coupled and should extend along these strips for a distancecomparable to the coupling region. Such a slot has been found toincrease the amount of energy coupled between adjacent strips by amountsof the order of one decibel. For a more complete understanding of thisincrease in coupling, reference may be had to FIGS. 2 and 3.

In FIG. 2 there is shown, in cross section, a strip type transmissionline coupler 20 comprising outer parallel ground planes 21, 22 andcenter conductive strips 23, 24 separated by dielectric slabs 25. Forpurposes of discussion, it will be assumed that strip 23 is the maintransmission path and is excited in the dominant TEM strip line wavemode. The electric field lines of such mode are illustrated in FIG. 2 assolid lines extending between strip 23 and planes 21, 22. As may readilybe seen, the field lines in the vicinity of the edges of strip 23 fanoutward in a fringing eifect and thereby extend beyond the centerline 26between the strips into the area which would be occupied by a dominantwave propagating on strip 24. By vintue of this extension of the fieldlines into the volume occupied by a wave on the adjacent strip, a Waveof low intensity is coupled onto line 24. The amount of coupling isdetermined by the separation of the parallel strips, the length of thecoupling section, and the width of the strips themselves.

FIG. 3 is a cross sectional view of a strip type transmission line 30comprising outer parallel ground planes 31, 32 and parallel strips 33,34 separated by dielectric slabs 35. Extending in ground plane 31 alongand above the region of coupling between strips 33, 34 is slot 36. As inFIG. 2, it is assumed that strip 33 is the main transmission line and isexcited in the dominant TEM wave mode. The approximate distribution ofthe electric field lines associated with this mode is designated by thesolid lines extending between strip 33 and the outer conductive groundplanes. Since the electric field lines terminate on conductive surfaces,slot 36 causes the normally symmetric wave pattern to be slightlydistorted. In particular, it may be seen that several of the illustratedelectric field lines extend well beyond centerline 37 into the volumeoccupied by a wave on strip 34. Since the electric field is thusconstrained to extend further into the region of coupling with strip 34,the amount of coupling from strip 33 is increased.

Returning now to FIG. 1, it has been found that the increase in couplingbetween the lines occurs for various transverse positionings of slot 17in the portion of ground plane 13 extending above the area between thestrips 11, 12. However, the optimum positions are those in which theslot is disposed on the side of the centerline between the strips whichis closer to strip 12, the coupled-o line. This optimum range obtainssince the impedance discontinuity represented by slot 17 is minimizedwhen the slot is positioned away from the main transmission strip, onwhich the higher power is propagating.

Mounted on plane 13 and positioned at least partially over slot 17 isconductive covering member 18. Member 18 may take the form of arectangular plate, as illustrated in FIG. 1, or it may be of otherconductive construction as will be described in connection with FIG. 4.In FIG. 1, plate 18 is slidably mounted between guiding lips 19, 19' andmay be variably positioned to cover a selected amount of slot 17. Whenthe desired position is attained, screw 50 in lip 19' may be tightenedagainst plate 18 to hold it in place. Electrically, plate 18 serves tocancel the effect of slot 17 in the area of covering by presenting anunbroken conductive surface to propagating waves on strip 11. Thus, thepropagating waves are not distorted to extend further into the region ofstrip 12. The amount of coupling between strips 11, 12 may, therefore,be easily controlled externally Within a given range by variablypositioning plate 18.

In the typical operation of the strip line coupler of FIG. 1, strip 11is excited in the dominant TEM wave mode by energy source 51. Thisenergy propagates along strip 11 and a substantial portion of thisenergy reaches utilizing means 52. When the vicinity of strip 12 isencountered, however, a portion of the energy on strip 11 is coupledonto strip 12 in the manner explained above in connection with FIGS. 2and 3. The coupled energy proceeds primarily to utilizing means 53.However, since the directivity of the coupler is not perfect, a smallamount of coupled energy may proceed in a direction opposite to the mainpropagation direction. Attenuator 54 is provided at the end of strip 12opposite utilizing means 53 to dissipate this reverse-coupled energy.The amount of energy coupled from strip 11 to strip 12 may be controlledwithin a specified range by cover plate 18. As stated above, thepresence of slot 17 increases the amount of energy coup-led onto strip12. Variations in the amount by which plate 18 covers, and thereforeelectrically neutralizes, slot 17 may be utilized to vary the amount ofcoupling provided.

FIG. 4 is a top view of a multiunit strip line directional couplerdevice 40 with respect to which the present invention has particularlyutility. Illustrated is a sandwich type main strip line comprising apair of outer ground planes 41 and a strip center conductor 42. Couplingcircuits comprising strip center conductors 43, 44, and 45 arepositioned in parallel path relationship with strip 42 over a portion ortheir lengths. The termini of each of strips 42-45 are fitted withcircularly coaxial connectors 46 for interconnecting the coupler into anexternal circuit. Located in top ground plane 41 and positioned betweeneach of strips 4345 and strip 42 are slots or apertures 47, 43, and 49,respectively. If the dimensions of the various strips as well as theirseparation are properly selected, the power level on each of the coupledcircuits may be in the range of one one-thousandth of the power in themain transmission circuit. That is, the couplers will be what is knownin the art as 30 decibel couplers. Slots 47-49 may then increase thecoupling of each coupler by 0.5 decibel. If a greater range of trimmingis desired, a second slot may be positioned opposite each of theillustrated slots on the lower conductive ground plane. The ground planeslot-s may be utilized for the purpose of equalizing the coupling amongthe three couplers. One adjustment procedure involves measuring thecoupling of each individual coupling circuit, thereby determining whichcircuit exhibits least coupling; and unro lling a strip ofadhesive-backed metallic foil such as copper over the slots associatedwith the other coupling circuits until the coupling of the latter twocircuits is decreased to equal the coupling of the first circuit. Afterthe equalization procedure is completed, the foil is cut and permanentlycemented in place. In FIG. 4 the circuit associated with strip 44 isshown to be that with least original coupling. Copper strips 55, 56partially cover slots 47, 49, respectively, to attain an amount ofcoupling of wave energy from strip 42 to auxiliary strips 43 and 45which is equal to that to strip 44.

In general, the above-described principles are applicable to any of thevarious symmetrical fiat strip type transmission lines in which acentral strip is sandwiched between and dielectrically spaced from apair of external conductive ground planes. In all cases, it is to beunderstood that the above-described arrangements are simply illustrativeof a small number of the many possible specific embodiments which canrepresent applications of the principles of the present invention.Numerous and varied other anrangements can readily be devised inaccordance with these principles by those skilled in the art withoutdeparting from the spirit and scope of the invention.

What is claimed is:

1. In combination, a first electromagnetic wave energy transmission pathcomprising a first flat conductive strip dielectrically spaced between apair of parallel conductive ground planes,

a second electromagnetic wave energy transmission path comprising asecond flat conductive strip dielectrically spaced between said pair ofground planes, said dielectric spacing between said pair of parallelplanes and said first and said second strips being continuous,

said first and said second strips positioned in the same plane intransverse coupling proximity with a finite dielectric spacing betweenthe adjacent edges of said strips,

and means for increasing the energy coupling between said first and saidsecond strip,

said means comprising an elongated nonconductive aperture in one of saidground planes in the region overlaying said finite dielectric spacingbetween said strips.

2. The combination according to claim 1 in which said aperture isdisposed to one side of the longitudinally extending centerline betweensaid strips.

3. The combination according to claim 1 including means for adjustingthe amount of coupling between said strips in the presence of saidaperture,

said adjusting means comprising a conductive element positionable tocover variable areas of said nonconductive aperture.

4. An adjustable strip line directional coupler unit comprising a pairof dielectrically spaced conductive planes in overlaying parallelrelationship,

a main electromagnetic wave energy transmission path comprising a firstconductive member extending be tween and dielectrically spaced from saidpair of conductive planes,

at least one auxiliary transmission path comprising a second conductivemember extending between said pair of conductive planes and dieleotn'cally spaced from said planes and from said first conductive member, thedielectric spacing between said conductive planes and said conductivemembers being continuous,

said auxiliary path positioned in the field of waves supported by saidmain path,

said conductive planes having at least one non-conductive aperture ofvariable area overlaying the dielectric region between said main andsaid auxiliary transmission paths, the amount of energy coupied fromsaid main transmission path to said auxiliary transmission path beinggreatest when the variable area of said nonconductive aperture ismaximized.

5. A strip line wave guide directional coupler for wave energy in atransverse magnetic mode comprising at least four conductive planarmembers extending coextensively parallel,

each of said members being dielectrically spaced firom each other ofsaid members along their coextensive portions by a continuous dielectricmedium,

two of said members being wider in transverse dimension than theremainder of said members and positioned to extend on opposite sides ofsaid remainder of members to sandwich said remainder of memberstherebetween,

each of said remainder of members being positioned in energy couplingrelationship with at least one other of said remainder,

said two wider members having at least one non-eonduetive aperturetherein superposed above the region of coupling between a pair of saidremainder of members,

and means for decreasing the coupling between said pair of saidremainder of members,

said means comprising a conductive member variably positionable oversaid aperture to render said aperture at least partially conductive.

References Cited in the file of this patent UNITED STATES PATENTS2,735,069 Riblet Feb. 14, 1956 2,829,347 Tomiyasu Apr. 1, 1958 2,901,709Fitzmorris Aug. 25, 1959 2,951,218 Arditi Aug. 30, 1960

1. IN COMBINATION, A FIRST ELECTROMAGNETIC WAVE ENERGY TRANSMISSION PATHCOMPRISING A FIRST FLAT CONDUCTIVE STRIP DIELECTRICALLY SPACED BETWEEN APAIR OF PARALLEL CONDUCTIVE GROUND PLANES, A SECOND ELECTROMAGNETIC WAVEENERGY TRANSMISSION PATH COMPRISING A SECOND FLAT CONDUCTIVE STRIPDIELECTRICALLY SPACED BETWEEN SAID PAIR OF GROUND PLANES, SAIDDIELECTRIC SPACING BETWEEN SAID PAIR OF PARALLEL PLANES AND SAID FIRSTAND SAID SECOND STRIPS BEING CONTINUOUS, SAID FIRST AND SAID SECONDSTRIPS POSITIONED IN THE SAME PLANE IN TRANSVERSE COUPLING PROXIMITYWITH A FINITE DIELECTRIC SPACING BETWEEN THE ADJACENT EDGES OF SAIDSTRIPS, AND MEANS FOR INCREASING THE ENERGY COUPLING BETWEEN SAID FIRSTAND SAID SECOND STRIP, SAID MEANS COMPRISING AN ELONGATED NONCONDUCTIVEAPERTURE IN ONE OF SAID GROUND PLANES IN THE REGION OVERLAYING SAIDFINITE DIELECTRIC SPACING BETWEEN SAID STRIPS.